1. Field of the Invention
The present invention relates to a method for forming a fine pattern which is used for preparing a semiconductor device or integrated circuit by forming the pattern using electron beam lithography.
2. Related Art
In a conventional method for preparing IC and LSI, a pattern is formed by photography using ultraviolet rays. With an increasing demand for finer elements, a stepper lens is being equipped with high numeral apertures and a light source of short wavelength has been used but on the other hand, these embodiments encounter a drawback of causing a shallow focus depth. Furthermore, a fine pattern size of LSI element and manufacturing ASIC necessitate to apply electron beam lithography to these techniques. Fine pattern formation by this electron beam lithography is essentially required for an electron beam resist. Inter alia, polymethyl methacrylate (PMMA) is a positive type electron beam resist and known to provide the best resolution; however, its sensitivity is poor. In recent years, many reports have thus been made to increase the sensitivity of a positive type electron beam resist, and there are known positive type electron beam resists comprising, for example, polybutyl methacrylate, a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methyl methacrylate and isobutylene, polybutene-1-sulfone, polyisopropenyl ketone, fluoropolymethacrylate, etc. These resists are all designed to easily cause scission of the principle chain with an electron beam by introducing an electron withdrawing group onto the side chain or introducing a readily degradable bond on the principle chain thereby to obtain high sensitivity. However, even these resists do not sufficiently satisfy both resolution and sensitivity. In addition, neither dry etch durability nor heat durability is sufficiently good so that these resists are used only with difficulty as masks for dry etching and have restricted application.
Turning to a negative type electron beam resist, the resist comprises a cyclized rubber and its dry etch durability is good. However, the resist involves drawbacks that close contact with a substrate is poor, it is difficult to obtain a uniform pinhole-free coated layer of high quality on the surface of a substrate, it has poor heat durability, poor resolution, etc. Therefore, various improvements have been hitherto made on negative type electron beam resists. There are proposed negative type electron beam resists comprising, for example, polyglycidyl methacrylate, chloromethylated polystyrene, chloromethylated .alpha.-methylpolystyrene, polymethacrylate maleic acid ester, polystyrene chloride, a copolymer of polyglycidyl methacrylate and ethyl acrylate, etc. These resists are all designed to easily generate a radical by an electron beam and cause crosslinking by introducing an epoxy group or a chlorine atom liable to reacting with an electron thereby to obtain high sensitivity. However, even these resists do not sufficiently satisfy both resolution and heat durability. In order to develop such a negative type resist comprising a substrate of cyclized rubber or isoprene and a thermoplastic polymer provided thereon, an organic solvent should be used so that the drawn resist might be swollen in an organic solvent developer upon development. Furthermore, such a mono component type resist involves a problem of post polymerization when the resist is allowed to stand after exposure. The polymerization proceeds with passage of time and as the time of allowing to stand after exposure is prolonged, the size of resist pattern after development is thickened as shown in FIG. 5C. Therefore, the resolution of the pattern is lowered and in some occasion, the pattern is distorted until it cannot be used any more. In addition, the organic solvent developer is harmful in view of environment and health and is undesirable also in view of flammability.
In recent years, a concept of chemical amplification has been introduced to enhance the sensitivity of a negative type electron beam resist and research and development have been made on such a resist. In the chemical amplification, a tri-component system substance comprising a photo acid generator capable of generating an acid upon exposure to an electron beam, a monomer which is reactive by the action of the acid and a novolak resin is used as a negative type electron beam resist. As the photo acid generator which can generate an acid upon exposure to an electron beam, there are an organic halide compound, an onium salt, etc. Examples of the organic halide compound include 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane, 1,1-bi(p-methoxyphenyl)-2,2,2-trichloroethane, 1,1-bis(p-chlorophenyl)-2,2-dichloroethane, 2-chloro-6-(trichloromethyl)pyridne, etc. Examples of the onium salt are triphenyl sulfonium, diphenyl iodonium, etc. These compounds generate a Lewis acid which is a strong acid upon exposure to an electron beam. Examples of the monomer which is reactive by the action of the acid include melamine and methylol melamine. Methylol melamine has the following chemical formula and reacts with the acid to release --OH group. ##STR1##
These compounds react with novolak resin which is a matrix polymer to cause the following crosslinking reaction: ##STR2##
The reaction described above proceeds to cause a three dimensional crosslinking of novolak resin. That is, by exposure to an electron beam, Lewis acid generates from the acid generator and the monomer such as melamine reacts with novolak resin by the acid to form the crosslinked structure. In order to proceed the crosslinking reaction, it is necessary to bake immediately after exposure. Where the system is allowed to stand without baking, the acid generated by exposure reacts with oxygen, etc. in the air and inactivated; as shown in FIG. 5B, these compounds involve a defect that the size of resist pattern after development becomes drastically thin, as the time from exposure to baking is prolonged.
Electron beam lithography encounter drawbacks of poor dry etch durability and poor heat durability of an electron beam resist, adverse influence on pattern precision by a proximity effect due to forward scattering or back scattering of electrons. In order to compensate for these drawbacks, a multilayer resist pattern in which the function of the-resist is shared into an imaging layer and a planarizing layer is extremely effective. FIGS. 6, A through D are to explain the process of a conventional tri-layer resist in electron beam lithography. In order to prevent the proximity effect, a high molecular organic layer is coated on substrate 11 in a thickness of 2 to 3 .mu.m as the bottom layer 21 (FIG. 6A). Further thereon an inorganic film of SiO.sub.2, etc. or an inorganic high molecular film of SOG (spin on glass), etc. is coated in a thickness of 0.2 .mu.m as an intermediate layer 22 and an electron beam resist is further coated on the intermediate layer in a thickness of 0.5 .mu.m as the top layer resist 23 (FIG. 6B). After a pattern is drawn with an electron beam 24, development is performed with a developer used exclusively for an organic solvent to obtain a resist pattern 23P (FIG. 6C). Next, the intermediate layer 22 is subjected to dry etching using this resist pattern 23P as a mask and the bottom layer 21 is then subjected to dry etching using this intermediate layer 21 as a mask to transfer the pattern (FIG. 6D). A fine pattern can be formed in a high aspect ratio through the foregoing multilayer resist process. In such a tri-layer resist process, however, steps are more complicated and defects frequently occur. Further where selectivity in etching in the intermediate layer and the bottom layer is small, there is a problem that a pattern size shift in transferring the pattern becomes larger than 0.1 .mu.m. Therefore, the tri-layer resist process is not practical, either.
As described above, the tri-layer resist process is an effective process but involves problems in complicated steps, deviation of resist size in transfer of a pattern, etc. In the case of electron beam lithography, an incident electron causes scattering inside the resist; the electron which reaches a substrate causes back scattering, and again returns into the resist to sensitize the resist. Since pattern precision is seriously deteriorated by such a proximity effect, it is necessary to provide a thick bottom layer to prevent the back scattering electron. Thus, a silicon containing resist for a bilayer resist process which has the function as a mask for the bottom layer and as a resist layer at the same time, an inorganic resist, etc. have been developed. There are, for example, a resist pattern having a siloxane bond on the principle chain, ladder type polysiloxane, chalcogenide glass type inorganic resist, etc. However, these resists cannot sufficently improve dry etching durability, are poor in sensitivity and resolution and are thus far away from practical use. In fact, sensitivity and resolution of these conventional resists are about 20 .mu.C/cm.sup.2 and about 1 .mu.m, respectively. Since an organic solvent is used as a development, these resists involve additional problems that change in sensitivity and change in size of such resists are large, process allowance is small, swelling occurs upon development and a resist pattern cannot be formed accurately. In addition, there are problems of environmental pollution, harmfulness to human, etc. In order to solve these problems, the present inventors have accomplished highly sensitive silicon-containing electron beam resists and a fine pattern forming method using these resists.